SUE Quality Levels Under AS5488: What QL-D, QL-C, QL-B, and QL-A Mean for Your NSW Project
Subsurface Utility Engineering (SUE) quality levels define how accurately underground utilities are known before ground is broken on any project. Under the Australian Standard AS5488, four quality levels, QL-D, QL-C, QL-B, and QL-A, classify the reliability of subsurface utility information from desktop records through to physical verification. Each level requires a different investigation method, produces a different confidence threshold, and applies to a different phase of project development. Selecting the wrong quality level for a given project stage is one of the primary causes of utility strikes, design conflicts, and construction cost overruns in NSW.
What SUE Quality Levels Are and How AS5488 Defines Them
AS5488 is Australia’s national standard for the classification and management of subsurface utility information (SUI). It establishes four quality levels, QL-D, QL-C, QL-B, and QL-A, each representing a different degree of accuracy, method of data collection, and confidence in the location of underground assets.
The standard does not treat these levels as interchangeable options. Each level builds on the data collected at the level below it. A project that proceeds to QL-B investigation without completing QL-D and QL-C first lacks the anomaly context that makes geophysical designation meaningful. A project that stops at QL-B, where the design stakes require QL-A, does so at the risk of a utility strike.
For civil engineers, geotechnical consultants, project managers, and excavation contractors working across NSW, understanding where each quality level applies and what its limitations are directly affects project safety, cost, and programme.
QL-D: Desktop Records Research
QL-D is the starting point for every subsurface utility investigation. At this level, utility information comes entirely from existing records, utility franchise maps, as-built drawings, GIS databases, permit records, and, where available, verbal input from long-term facility owners familiar with the site.
QL-D data has the lowest reliability of the four quality levels. Records are frequently incomplete, outdated, or reflect routes that were altered after original construction without formal documentation. Memory-based input introduces further uncertainty. Under AS5488, QL-D provides no spatial tolerance on utility location.
QL-D applies to project feasibility studies, preliminary route selection, and area-wide planning in which underground utilities are among the many variables under review. No project should proceed to active design or excavation based on QL-D data alone.
QL-C: Surface Feature Survey
QL-C advances beyond desktop research by correlating QL-D records with a physical survey of visible above-ground utility features. Manholes, access chamber lids, stop valves, hydrants, gas risers, and power pole dip points are located, surveyed, and mapped against the existing record data.
This process frequently reveals discrepancies, utilities shown on plans that have no corresponding surface feature, or surface features with no record equivalent. These anomalies are precisely the information that shapes the scope and target areas of the subsequent QL-B investigation.
QL-C does not provide underground utility location data. Surface features indicate where a utility enters or exits the subsurface domain; they do not confirm the path between those points. Under AS5488, QL-C carries no spatial tolerance on the horizontal alignment of buried assets between surface access points.
QL-C applies during initial design phases where a broad utility context is required before geophysical investigation is commissioned.
QL-B: Geophysical Designation Using GPR and EMI
QL-B is the most widely used quality level in active design and construction planning. At this level, surface geophysical methods are applied to detect the horizontal position of underground utilities across the project area without excavation.
Under AS5488, QL-B data carries a spatial tolerance of ±300mm horizontal and ±500mm vertical. Two primary technologies deliver QL-B results:
Electromagnetic Induction (EMI) transmits a signal along electrically conductive utilities, power cables, metallic pipes, telco lines, and traces their horizontal position at the surface. Smartscan Locators uses Radiodetection RD8000 and Vivax Vloc3 electromagnetic locators for QL-B designation, along with Flexi Trace rods to depths up to 150m and sondes to a tested depth of 8m for non-connected conduits.
Ground Penetrating Radar (GPR) emits high-frequency radar pulses into the ground and captures reflections from subsurface changes in material, including non-conductive utilities such as plastic pipes, fibre optic conduits, and concrete drainage lines that EMI cannot detect. Smartscan operates Mala and Leica GPR systems for GPR survey work across NSW.
QL-B answers the question of what utilities are present and approximately where they run horizontally. It does not confirm depth with precision, and it does not validate the physical state of what was detected. Where QL-B results conflict with existing records, a signal appearing where no utility is documented, or a documented utility producing no signal, a QL-A investigation is warranted at those specific points.
QL-A: Physical Verification Through Potholing
QL-A is the highest quality level under AS5488 and the only level that produces validated utility data. A utility is classified QL-A only after physical exposure at a specific point, with its absolute 3D position confirmed through direct measurement.
AS5488 specifies a maximum spatial tolerance of ±50mm for QL-A data, a precision threshold achievable only through actual exposure, not geophysical inference. In addition to location, QL-A captures utility attributes: owner, type, status, material, size, and conduit configuration. This metadata is recorded with the date of data capture, the survey and locating methods used, and the survey control information used to establish absolute spatial position.
Physical exposure at QL-A is achieved through non-destructive digging (NDD). Smartscan’s vacuum truck NDD crew uses hydro-vacuum excavation to remove soil around subsurface utilities without applying mechanical force to the asset. Vacuum trucks in the Smartscan fleet carry between 6,000L and 9,000L and are equipped with dual gerni lances to reduce excavation time in congested areas.
QL-A investigation is targeted; it is applied at specific critical points identified during QL-B designation, not across an entire project corridor. High-risk crossings, deep foundation zones, and areas where proposed construction comes within proximity of high-voltage cables, gas lines, or fibre optic infrastructure are the primary trigger points for QL-A potholing.
Why Quality Levels Must Be Applied in Order
AS5488 quality levels are not a menu. They are a progressive investigation sequence: QL-D → QL-C → QL-B → QL-A.
The reason is operational, not administrative. QL-D and QL-C data identify the anomalies and record inconsistencies that determine where QL-B geophysical investigation should focus. QL-B designation maps the utility conflicts and depth uncertainties that determine exactly where QL-A potholing should be placed.
Commissioning QL-A potholing without prior QL-B investigation is the equivalent of concrete scanning before coring, it is possible to core without scanning, but the absence of prior scan data means test holes are placed by assumption rather than by signal. The cost of misplaced QL-A test holes exceeds the cost of conducting QL-B first. The sequence exists to make each investigation phase targeted and efficient.
When Each Quality Level Is Required on NSW Projects
The project phase determines which quality levels apply:
Feasibility and route selection: QL-D and QL-C provide the desktop and surface-feature baseline required to assess utility congestion and flag areas requiring closer investigation.
Preliminary and detailed design: QL-B geophysical designation provides the horizontal utility map needed to route new infrastructure around existing assets and identify conflict zones before design is locked.
Pre-construction and critical crossings: QL-A potholing is required at any point where excavation depth brings construction within the tolerance zone of a known or suspected utility, and at all high-risk crossings, high-voltage cables, gas transmission lines, telco ducts, where a strike carries significant safety and financial consequences.
For utility locating across NSW to meet AS5488 compliance, the quality level assigned to each utility on the investigation deliverable must reflect the method used to obtain it, not the method the operator intended to use.
When QL-A Physical Exposure Is Not Achievable
QL-A is the highest confidence level available, but it cannot be achieved in all field conditions. Assets installed through trenchless boring in rock formations, utilities encased in concrete at significant depth, and infrastructure placed by under-boring technology at depths beyond the reach of vacuum excavation equipment may not be exposed through NDD methods.
Where QL-A cannot be achieved, AS5488 requires that the quality level assigned to that utility reflect the actual data confidence, typically QL-B, rather than the intended classification. A utility that cannot be physically verified does not receive a QL-A classification regardless of the quality of the surrounding data.
What a SUE Deliverable Looks Like Under AS5488
A compliant AS5488 investigation report assigns a quality level to every utility identified within the project area. Each utility is shown with the classification level, the method used to obtain it, the date of data capture, and, at QL-A, the full attribute set including type, material, size, status, and owner.
Smartscan produces utility mapping and reporting deliverables structured to AS5488 classification requirements, providing project teams with documentation that can be attached directly to excavation permits and retained on file for future reference. Reports include visual data on located assets, dimensions, depths, and connecting infrastructure, formatted to support both design integration and site safety protocols.
Frequently Asked Questions
What is the difference between QL-B and QL-A?
QL-B uses surface geophysical methods, GPR, and EMI, to determine the approximate horizontal position of underground utilities without excavation. QL-A requires physical exposure through potholing to confirm the precise 3D position, material, size, and condition of a utility at a specific point. QL-B locates; QL-A validates.
Is QL-B sufficient for construction design in NSW?
QL-B is sufficient for most preliminary and detailed design work where utilities need to be routed around. At points where construction comes within close tolerance of a critical utility, or where depth data is required for deep foundation or trenchless works, a QL-A investigation is required at those specific locations.
What does AS5488 require for a utility to be classified QL-A?
Under AS5488, QL-A classification requires physical exposure at a point, measurement of absolute 3D spatial position within ±50mm, and capture of utility attributes including type, status, material, size, and configuration. Metadata must include the date of capture, the locating methods used, and survey control information.
Do quality levels apply to all utility types?
QL-B and QL-A apply to both conductive and non-conductive utilities. EMI is effective on metallic pipes and cables with a tracer wire; GPR detects non-conductive assets, including plastic pipes, fibre optic conduits, and concrete drainage. A thorough QL-B investigation uses both methods to account for the full range of buried asset types.
What is the AS5488 tolerance for QL-B data?
AS5488 specifies ±300mm horizontal and ±500mm vertical spatial tolerance for QL-B geophysical designation data.
Smartscan Locators delivers AS5488-compliant underground utility investigation across NSW, from QL-B geophysical designation using Radiodetection RD8000 and GPR systems through to QL-A physical verification with NDD vacuum excavation. For project-specific advice on the correct quality level scope for your investigation, contact the Smartscan team directly.